WASHINGTON (Reuters) - The bombmaker suspected of designing exotic weapons like underwear bombs for al Qaeda's Yemeni affiliate is a key target of U.S. counter-terrorism efforts, a top U.S. official said on Wednesday.

Matthew Olsen, director of the National Counterterrorism Center, in his first public comments since a plot by Al Qaeda in the Arabian Peninsula, or AQAP, was recently foiled said the Yemeni affiliate was the most dangerous.

U.S. officials have said AQAP was behind a recent plot to arm a suicide bomber with an improved version of the underwear bomb that failed to explode on a U.S.-bound flight on Christmas Day 2009.

The device, which is being examined by the FBI, bore the hallmarks of Ibrahim Hassan al-Asiri, a Saudi militant who is believed to be a bombmaker working with AQAP, U.S. officials say.

"Al-Asiri is a person who has been identified as one of the bombmakers for al Qaeda in Yemen, Al Qaeda in the Arabian Peninsula, and somebody who has proven to be a proficient bombmaker and to have tried to adapt devices to the types of security measures that it seems we have in place," Olsen said.

"He is a very important person for us to find out where he is and to take appropriate action," he said.

The latest underwear bomb plot from Yemen, revealed last week, was a reminder that al Qaeda's affiliates remain a "persistent" and dangerous threat, Olsen said at a breakfast meeting of the American Bar Association's law and national security committee.

"Tremendous gains" against al Qaeda by the United States and its allies, particularly in Pakistan, have left the group under siege with inexperienced leaders who are having trouble recruiting, training, and communicating, Olsen said. A key breakthrough was the killing last year of al Qaeda leader Osama bin Laden by U.S. commandos at his hideout in Pakistan.

"In short, the intelligence picture shows al Qaeda core is really a shadow of its former self. The overall threat from the tribal areas of Pakistan has been greatly diminished," he said.

While the core al Qaeda group which bin Laden once led struggles to "remain relevant," it is looking toward affiliates in Yemen, Somalia, Algeria, Iraq, and Iran, to carry out attacks and advance its ideology, Olsen said.

AQAP in Yemen is the "most operationally active" and the most dangerous, he said. "The group is still looking for opportunities to attack us here in the United States, and we saw this vividly last week with the foiled plot to use an explosive device to take down an airliner," Olsen said.

AQAP also seeks to influence people outside Yemen and to encourage extremists in the United States to carry out attacks on their own, he said.

Olsen said he would not talk specifically about any leak investigation related to the Yemen operation. But in general, he said, leaks "are devastating" and can compromise the way intelligence is collected.

"It's not an exaggeration when somebody says that leaks do endanger people's lives, that is not an exaggeration," Olsen said.

The Office of the Director of National Intelligence is conducting an internal review of whether any classified information about the recent plot leaked from the intelligence agencies. The FBI is also conducting a criminal investigation.